WASHINGTON, Dec. 12 (Xinhua) -- The U.S. Department of Defense said here on Wednesday in a rare public pushback against Turkey's operation plan in Syria that any unilateral military action in northeast Syria would be "unacceptable."

Earlier on Wednesday, Turkish President Recep Tayyip Erdogan said that his country will launch a military operation in the east of Euphrates River in Syria in a few days.

He also blasted the United States for establishing observation posts along Syria's border with Turkey despite Ankara's opposition.

"It is clear that the purpose of the U.S. observation points in Syria is not to protect our country from terrorists but to protect terrorists from Turkey," Erdogan said.

Commander Sean Robertson, a Pentagon spokesman, said hours later in a statement that "unilateral military action into northeast Syria by any party, particularly as U.S. personnel may be present or in the vicinity, is of grave concern. We would find any such actions unacceptable."

The statement added that consultation between the United States and Turkey, a U.S. NATO ally, "is the only way to secure the border area in a sustainable manner, and ... that uncoordinated military operations will undermine that shared interest."

While Washington has been fully committed to Ankara's border security, it will also work with the Syrian Democratic Forces, which includes Kurdish YPG fighters, "to ensure ISIS's enduring defeat," it said.

Ankara has long regarded the YPG as Syrian affiliate of the Kurdistan Workers' Party that has been designated as a terrorist organization by Turkey, the United States and the European Union, arguing that its presence along Turkey's southern border poses a serious threat to its national security.

Turkish and U.S. forces last month started joint patrols in the Manbij region of northern Syria. In a separate move, U.S. Defense Secretary James Mattis said on Nov. 21 that the U.S. military is building observation posts along the Turkey-Syria border, in an effort widely seen to reduce tension with Turkey.

However, such efforts appeared to have been made in vain after the Turkish Armed Forces fired artillery shells at YPG positions in the east of the Euphrates in northern Syria last month.